The evidence is also against there being such a thing as ladies synchronizing their menstrual cycles. I know. I know. It totally happened to you in college. I thought it happened to me, too. And there are few scientists who think the phenomenon is real. But the preponderance of evidence seems to be against them. Again (and this cannot be said enough) humans are really good at spotting patterns—even when patterns don't exist.

Maybe we should look to our primate relatives for evidence, then: in fact two papers have come out this year testing this hypothesis in primates! Setchell et al (2011) observed semi free-living mandrills, which is a kind of Old World monkey, a group to which the Great Apes belong. Out of ten observation-years of data, they found a single year that had significant synchrony… only to have that one year fail to be significant once they corrected for multiple testing. Multiple testing corrections are important because of the chance that if you test a hypothesis enough times you will get a spurious significant result (and for a brilliant take on this, see this xkcd comic).

The other equally interesting paper to come out this year on this topic is by Fürtbauer et al (2011), entitled “You Mate, I Mate: Macaque Females Synchronize Sex Not Cycles.” Their study population was wild Assamese macaques, also Old World monkeys. Fürtbauer et al (2011) observed behavioral receptivity and measured fecal ovarian hormones (yes, that means they measured hormones in poop) in order to assess behavioral and hormonal synchrony. They found long periods of behavioral receptivity that synchronized well across individuals, but that actual estrus cycles were randomly distributed within the receptive period. I thought this paper did a great job at providing an evolutionary framework for why mating might evolve to be synchronized, but not cycles, and because the paper was published in PLoS ONE, you can read it yourself for free.

This paper resolves a question I’ve had for a long time about menstrual synchrony, which is how in the world it could actually be beneficial to females, particularly those with covert ovulation. Why would you want all the females, or even a subset of them, to be fecund and receptive at the same time? And the answer is, you probably wouldn’t. Humans, other primates, even some cetaceans like dolphins have mating that is largely decoupled from reproductive cycling. That is, we don’t only mate at the time in our cycle when our chances are highest to conceive, though we might find ourselves slightly more proceptive or receptive at that time. Sex is not just about making babies, but is an affiliative behavior, promoting bonding but also plain old enjoyment. Instead, it may in some circumstances make sense to have extended periods of synchronous receptivity, as within a promiscuous species like the Assamese macaques (Fürtbauer et al. 2011). But this isn’t necessarily an adaptive feature of the entire primate lineage.